1. Field of the Invention
The present invention relates to a flyback boost circuit which boosts an input voltage by using a transformer, and which charges and outputs the boosted voltage to a load device, and a strobe device using the flyback boost circuit. The present invention particularly relates to the detection of the wiring state of an output voltage detection terminal on the secondary side of the transformer or a grounding terminal of a secondary winding of the transformer.
2. Description of the Related Art
FIG. 6 illustrates a circuit configuration of a flyback boost circuit 300 of a related art example. When a clock signal output from a logic circuit 320 is supplied to the gate of an N-channel FET 322, the N-channel FET 322 repeats an ON-OFF operation. When the N-channel FET 322 is turned on, current flows through the primary side of a transformer 341. During this period, current does not flows through the secondary side of the transformer 341. Thus, energy is accumulated on the secondary side of the transformer 341. When the N-channel FET 322 is turned off, the energy accumulated on the secondary side of the transformer 341 is discharged, and a secondary current flows. Thus, charge is accumulated in a load capacitance 343 via a diode 342.
A terminal on the secondary side of the transformer 341 is provided with an output voltage detection terminal T4. The charge voltage detected by the output voltage detection terminal T4 during each boost operation is supplied to an operational amplifier 321 via charge detection terminal wiring 350. The operational amplifier 321 compares a reference voltage E3 output from a reference power supply 330 with the charge voltage supplied via the charge detection terminal wiring 350. If the charge voltage exceeds the reference voltage E3, the output of the clock signal from the logic circuit 320 is stopped to stop the boost operation. That is, the flyback boost circuit 300 controls the clock signal by detecting the charge voltage of the transformer 341, to thereby prevent the charge voltage (charge) accumulated in the load capacitance 343 from exceeding a predetermined value.
As described above, the related art flyback boost circuit 300 illustrated in FIG. 6 detects the increase of the charge voltage to a predetermined voltage by confirming the rise of the voltage of the output voltage detection terminal T4 to the target voltage, and controls the charge voltage not to exceed a predetermined value.
Further, as another related art example, in the capacitor charging device according to Japanese Unexamined Patent Application Publication No. 2003-59689, a technique is disclosed which provides a charge detection terminal on the output side of a transformer to detect a secondary current of the transformer and control the oscillation mode during the charging operation, to thereby reduce the time loss during the charging time. The above-described related art examples, however, do not disclose a technique of detecting an abnormality of the detection operation by determining the connection (alternatively described as wiring) failure of the output voltage detection terminal T4 or the wiring state of a grounding terminal of a secondary winding of the transformer.
In the flyback boost circuit 300 and the capacitor charging device described above, therefore, the wiring failure of the output voltage detection terminal T4 or the unwired state or the wiring failure of the grounding terminal of the secondary winding of the transformer 341 occurs in some cases due to a mounting failure or the like. In this case, an input voltage is excessively boosted, and the charge voltage exceeds a set value. As a result, disadvantages such as the deterioration of a load device or the like are caused.